In the continuous casting method of manufacturing steel, molten metal is cast directly into thin strip by a casting machine apparatus, the shape of the strip is determined by the mold of the casting machine apparatus. The strip may be further subjected to cooling and processing upon exit from the casting rolls.
In the twin roll caster, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls which are internally cooled so that metal shells solidify on the moving casting roll surfaces, and are brought together at the nip between the casting rolls to produce a thin cast strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish in a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.
The setting up of an adjustment of the casting rolls in a twin roll caster is a significant feature. The casting roll must be accurately set to fully define an appropriate separation of the casting rolls at the nip, generally of the order of a few millimeters or less. There must be some device for allowing at least one of the casting rolls to move outwardly against the biasing force to accommodate fluctuations in strip thickness, particularly during start-up.
Usually, one of the casting rolls is rotatably mounted in a fixed journal, and the other roll is rotatably mounted on supports that can move against the action of biasing force to enable the rolls to move laterally to accommodate fluctuations in casting roll separation and strip thickness. The biasing force may be supplied by helical compression springs, or alternatively, by a pair of cylindrical pressure fluid units. A strip caster with spring biasing of the lateral movement of the casting rolls is disclosed in U.S. Pat. No. 6,167,943 to Fish et a1.
Previously, we have proposed that the biasing force be substantially the same or slightly more than that required to balance the ferrostatic pressure of the casting pool and the mechanical friction involved in the moving of the casting rolls biasing toward each other such that a substantially constant gap is maintained between the rolls at the nips sufficient to provide separation between the solidified shells at the nip. This has been described in U.S. Pat. Nos. 6,536,506 and 6,988,530 issued on Mar. 25, 2003 and Jan. 24, 2006, respectively, in this prior casting roll method, the roll separation force is between 0 and 1.25 kN biasing force on each chock at the end of each casting roll. Or stated another way, a roll separation force is produced that is between about 0 to 1.85 N/mm across the casting roll surface. We now have found, surprisingly, that a slightly larger roll separation force is more effective, particularly in controlling the quality of the strip in connection with the invention described in Application Ser. No. 11/467,652, and in allowing strip thickness to be further reduced during a casting campaign, if desired.
It has also been proposed to have a roll separation force at the casting rolls between 5 and 150 N/mm. See U.S. 2005/0205233 A1, Sep. 22, 2005 and US 2005/0211412, Sep. 29, 2005. This roll separation force is not believed to allow for the type of control to provide quality thin cast strip as the present invention facilitates.